1. Field of the Invention
The present invention relates to transmit power amplification in general, and more specifically to radio frequency transmit power amplifiers with envelope feedback.
2. Description of the Related Art
High performance communication systems, such as those specified by high data rate IEEE 802.11 wireless standards, may use communication methods with high dynamic range signals. One example communication method known as Orthogonal Frequency Division Multiplexing (OFDM) may produce high dynamic range signals with high peak to average ratios (PARs). Transmit power amplifiers, which may be used to amplify signals before transmission, may need to operate with good linearity over a wide dynamic range of input signals to accommodate a signal's high PAR. Envelope detection may be used to extend the range of good linearity of a transmit power amplifier as described in co-pending application Ser. No. 12/388,477, filed on Feb. 18, 2009, entitled “Multiple Stage Envelope Detector”, by Manolis Terrovitis.
FIG. 1 illustrates a prior art linearized transmit amplifier 100 with envelope feedback using two envelope detectors 104. A variable gain power amplifier 103 may be adjusted continuously by a low frequency analog control signal Vc. The control signal Vc may be generated by a baseband comparator amplifier 106 that compares an envelope of an input signal RFIN with an envelope of an output signal RFOUT attenuated by a capacitive attenuator 107 with gain GCAP,ATT. The two envelope detectors 104 may be used to extract the envelope from the input signal RFIN and from an attenuated version of the output signal RFOUT. Because of the feedback loop provided by the envelope detectors 104, a gain GPA of the linearized power amplifier 103 may be forced to equal the inverse of the gain GCAP,ATT of the capacitive attenuator 107. Using envelope detection feedback seeks to improve the linearity of the transmit power amplifier 103.
FIG. 2 illustrates an input/output relationship 200 of an input Vi,PA and an output Vo,PA of a power amplifier based on a prior art “Rapp” model. The shape of the input/output curve in FIG. 2 may be represented by Equation (1).
                              V                      o            ,            PA                          =                  K          ⁢                                    V                              i                ,                PA                                                                    (                                  1                  +                                      V                                          i                      ,                      PA                                                              2                      ⁢                                                                                          ⁢                      R                                                                      )                                            1                /                                  (                                      2                    ⁢                                                                                  ⁢                    R                                    )                                                                                        (        1        )            The variables Vi,PA and Vo,PA in Equation (1) may indicate input and output amplitude voltages respectively, K may represent a linear signal gain, and R may indicate a “Rapp” coefficient. For a Rapp coefficient of R=1, as the input Vi,PA increases above a certain level, the amplifier may begin to saturate, thus introducing gain compression where the slope of the input/output curve changes from a constant linear value. Gain compression may also be known as amplitude to amplitude modulation distortion. For increasing Rapp coefficient R values, the input/output relationship in the Rapp model approaches an ideal R=∞ dashed curve and may have a relatively longer region of constant linear gain.
In a typical application, such as an 802.11 wireless transmitter, a transmit power amplifier may be required to “back off” its output power from a maximum saturation power level to ensure a minimum level of performance with an acceptable error vector magnitude. The amount of “back off” required for acceptable performance based on a Rapp model amplifier may depend on the Rapp coefficient. A transmit power amplifier in an 802.11g transmitter operating at 54 Mbps with a Rapp coefficient R=1 may be required to back off by 9.1 dB of power, while an ideal transmit power amplifier with a Rapp coefficient R=∞ may only need to back off by 5.4 dB. An amplifier operating with less power back off may be more power efficient. Thus it is desirable to apply a linearization method to a power amplifier that may result in an input/output relationship with increased linear dynamic range approaching the ideal R=∞ dashed curve in FIG. 2.
FIG. 3 illustrates an input/output relationship 300 for a prior art single stage envelope detector, with a linear dynamic range 307 from Vi,LO to Vi,HI that may typically extend over 12 to 15 dB of input signal levels. When the input signal RFIN is small, corresponding to Vi,ED=Env{RFIN}<Vi,LO, the envelope Vi,ED may be difficult to detect, in which case the linearization feedback loop may not be closed. In this “open loop” case, the control signal Vc into the transmit power amplifier 103 may be not well defined, and the loop gain GPA of the transmit power amplifier 103 may be significantly different from the desired value of 1/GCAP,ATT as shown in FIG. 4. This abrupt gain change may impact the performance of systems that may transmit signals with low level amplitudes. In particular, an OFDM signal for an 802.11 g transmitter may have both a high PAR and a trajectory of the in-phase and quadrature components of the signal that may pass through the origin. Thus, an OFDM signal may have an envelope with both high levels and an instantaneous zero level. Extending the linear dynamic range of a linearized transmit power amplifier over which closed loop operation may operate may provide a desired performance improvement.